No worries! I write a lot of questions too, that´s what the forum is for.
The increased price comes from the extra inputs for subwoofers, indeed; but Anthem has also improved the DAC´s on the 90 and it includes HDMI 2.1 (which is a must if you play videogames). The problems with the HDMI boards plagued early adopters such as Sound United (Denon & Marantz) during 2021; now in 2022 those issues seem to have been solved considering how manufacturers like Mcintosh, Denon, Marantz, or Yamaha have moved towards hdmi 2.1.
In the case of the AVM´s from Anthem, the extra price on the 90 includes the board while for the 70, you still have to pay for the upgrade. Is it worth the price increase? That is up to you, specially considering how the AVM70 is an excellent device (tested by Amir here in ASR!) at a super competitive price, but on a personal note, yes, I think the price increase is a bit too much.
Arc Genesis, the room equalization system for Anthem seems to be excellent after a year with a gazillon bugs (it seems that they have been fixed at the current moment). It can also integrate subwoofers with more ease than Dirac or Audyssey, but I cannot confirm it since I haven´t tried myself. With Audyssey, I can tell you that it is possible, but takes time and effort.
For years, AVR´s only had one subwoofer output. The trick to "duplicate" (or triplicate, cadruplicate...) it was using a miniDSP to plug the subwoofers to and adjust from there, then, run the calibration of the AVR once you have your subwoofers adjusted. It works, but again, it takes time and effort. That´s how you do it on a Denon without four subwoofer outputs.
Arc, Audyssey, Lingdorf, Dirac or any other equalization program will change how your speakers, Monitor Audio or any other manufacturer, perform. Do not expect miracles, though. All those systems adjust to an extent parameters like timing, phase, and most importantly, frequency reproduction. To a huge extent? Some more agressively than others, sure, but not to the point of making, let´s say, a Kef speaker, sound like a Bowers speaker.
In my experience, it is a better idea to use speakers with a performance in frequencies as flat as you can while you adjust the acoustics of your room to reduce peaks and dips in frequencies and then, once that´s done, run eq and fine tune your system.
And that is perhaps the crucial element. If the building is taking time, no matter what electronics and speakers you use, they will benefit from good acoustics in all cases. Since time is on your side, you can wait for better prices or new products on electronics and speakers.